It definitely matters what the task and aforementioned 'theme' is. I believe that for most knowledge, if one uses the Zettelkasten method, or any of its relatives, it's quite important to have 'large' collections. For non-Zettelkasten-esque methods, I can't say I have much experience.
I keep my work-related research, my mathematics research, my software development notes, and my meeting notes in one Obsidian vault. I keep a Kanban board for my project in a file, which acts as a general guidepost for 'what do I do next?'

I also keep a 'table of tasks' note that automatically collects all my TODOs from my vault and lists them based on when they're due. One big issue with this setup is that it's not capable of connecting to any form of notification service, and give me reminders of what tasks need to be done. As a result, I keep my personal TODO lists on Google Tasks / Tasksboard, and if I have anything important work-wise, I also use a work-related task list. I am still in the process of trying to de-Google my personal computing, so I plan on eventually replacing Google Tasks with something self-hosted.
For personal research, journaling, and any form of sysadmin administration I need to do for my home network (writing down IP tables, processes, documenting my setup) I like to use Logseq. It's an Obsidian-like knowledge manager project, but it's FOSS. It's still quite early in its development, so I don't want to use it for anything that requires a lot of extensions, but I hope the project will grow and eventually bloom into a new standard for non-linear note taking.
All of this, of course, is also available on my phone, so I'm never really without the means to write down some quick notes.